Designing a large rotor blade of a wind turbine is limited by several factors. Regarding long and slender rotor blades, one of the most important design challenges is the clearance between the tip section of the rotor blade and the surface of the tower of the wind turbine during extreme load situations. This is to avoid collision between the rotor blade and the tower as the rotor blade typically features certain flexibility. The problem of maintaining a sufficient rotor blade tower clearance is usually solved from a structural design perspective. In particular, this involves an increase of the stiffness of the rotor blade by increasing the stiffness of the main spar of the rotor blade.
However, this is a purely structural solution which leads to a higher mass of the rotor blade and thus leads to a higher cost of the rotor blade as well as higher fatigue loads in both the rotor blades and the hub components of the wind turbine.
Aerodynamically, the problem of high tip deflections has been solved in the following ways:
1) Increasing the slenderness of the blade. The drawback of this solution is that the aerodynamic performance during normal operation of the wind turbine is also affected and that the structural building height of the wind turbine may be significantly reduced.
2) Reduction of the aerodynamic twist of the rotor blade towards the tip section. The drawback of this solution is the loss of performance during normal operation.
3) Use of active flaps. The drawback of this solution is the need to introduce active elements in the rotor blade. This is, for instance, problematic as the lifetime of a rotor blade may easily exceed 20 or 25 years and that servicing these active elements may be complex and costly.
Thus, it is desirable to provide ways to reduce the deflection of a rotor blade of a wind turbine such that a sufficient clearance between the rotor blade and the tower of the wind turbine is maintained and collision between the rotor blade and the tower is avoided.